The present invention relates to a method of forming a porous film used as, e.g., an inter-layer dielectric in a semiconductor integrated circuit device.
As the integration density of a semiconductor integrated circuit has increased, an increased wiring delay time resulting from an increase in wire-to-wire capacitance, which is a parasitic capacitance between metal wires, has presented an obstacle to the implementation of a semiconductor integrated circuit with higher performance. The wiring delay time is a so-called RC delay which is proportional to the product of the resistance of the metal wire and the wire-to-wire capacitance.
To reduce the wiring delay time, therefore, it is necessary to reduce the resistance of the metal wire or the wire-to-wire capacitance.
As a method of reducing the wire-to-wire capacitance, the reduction of the dielectric constant of an inter-layer dielectric formed between the metal wires has been considered. As an inter-layer dielectric having a low dielectric constant, a porous film has been under study as a replacement for a conventional silicon oxide film. It can be said that the porous film is only the film capable of providing a dielectric constant of 2.0 or lower.
In view of the foregoing, there have been proposed various methods of forming porous films.
As a first conventional method of forming a porous film, there has been known one wherein a solution of a siloxane polymer precursor containing a thermally unstable organic component is prepared and coated on a substrate to form a coated film, which is then subjected to a thermal process for decomposing and eliminating the organic component such that numerous fine holes are formed in hollow portions from which the organic component has eliminated.
As a second conventional method of forming a porous film, there has been known one wherein a wet gel is formed on a substrate by coating a silica sol solution on the substrate or by performing CVD and then the condensation reaction of the silica sol is caused in the wet gel, while the volume reduction of the wet gel is suppressed by controlling the speed at which the solvent eliminates from the wet gel, thus forming the porous film.
As a third conventional method of forming a porous film, there has been known a method wherein a solution of silica fine particles is coated on a substrate to form a coated film, which is then sintered such that numerous fine holes are formed between the adjacent silica fine particles.
However, the first conventional method has the problem of higher cost since it is necessary to prepare the solution of the siloxane polymer precursor. Moreover, since the coated film is formed by coating the precursor solution on the substrate, the amount of silanol remaining, in the coated film is increased to cause such problems as a degassing phenomenon which is the elimination of moisture or the like in a thermal process step performed subsequently and the degradation of the porous film resulting from the absorption of moisture by the film.
On the other hand, the second conventional method has the problem of higher cost since it requires a special coating apparatus for controlling the speed at which the solvent eliminates from the wet gel. Moreover, since a large number of silanol groups remain on the surfaces of the fine holes, they may cause serious degradation of the film because of high moisture absorption, unless they are removed. It is therefore necessary to silylate the surfaces of silanol groups, resulting in a complicated process. In the case of forming the wet gel by CVD, a special CVD apparatus different from a plasam CVD apparatus used normally in a semiconductor process is also required, which also increases cost.
In accordance with the third conventional method, the diameters of the fine holes formed between the adjacent silica fine particles are determined by a geometric configuration in which the silica fine particles are deposited so that the diameters of the fine particles are increased significantly. Accordingly, it is difficult to adjust the dielectric constant of the porous film to 2 or less.